A silicon wafer, which is obtained by processing a silicon single crystal grown by mainly CZ method into wafers, is used as a wafer for a substrate material on which a semiconductor device is formed. In growth of a silicon single crystal using CZ method, generally a seed crystal having a shape shown in FIG. 2(A) or 2(B) is gently brought into contact with a silicon melt heated to 1420° C. of a melting point or more, and then the seed crystal is gradually pulled upward from the melt when a temperature of the seed crystal becomes steady. Thereby, a silicon single crystal is grown below the seed crystal. At this time, because innumerable slip dislocations are generated in the seed crystal as a result of thermal shock arising when the seed crystal is brought into contact with the silicon melt of high temperature, a necking portion where a diameter of the crystal grown following the seed crystal is once gradually lessened to about 3–5 mm is formed as shown in FIG. 4 in order to eliminate the slip dislocations. When the slip dislocations can have been eliminated from the grown crystal, a diameter of the crystal is gently enlarged to a desired diameter (formation of an enlarging diameter portion), and then an approximately cylindrical silicon single crystal having a diameter required in a constant diameter portion thereof is pulled.
A method of eliminating slip dislocations generated when a seed crystal comes into contact with a silicon melt by lessening a diameter of the crystal to about 3–5 mm is called as Dash Necking method, which is a manufacturing method widely utilized when growing a silicon single crystal by CZ method.
On the other hand, in recent manufacture of a silicon single crystal, a type of production in which a constant diameter portion of the single crystal is lengthened as much as possible is adopted in order to improve a productivity of a silicon single crystal itself and a silicon wafer having a large diameter is required with the aim of obtaining a large semiconductor device and improving a yield. Therefore, a single crystal to be pulled is becoming larger in diameter and heavier in weight.
In cases that such a heavy silicon single crystal having a large diameter is grown, there naturally occurs a limit to the production using Dash Necking method in which a diameter of a neck portion is lessened to 5 mm or less, otherwise slip dislocations can not be eliminated.
Therefore, recently a method of growing a silicon single crystal free from dislocation without using Dash Necking method is also being studied. For example, in Japanese Patent Application Laid-Open (kokai) No. 10-203898, a technique in which a silicon single crystal is grown without forming a neck portion by using a seed crystal having a tip end with a sharp-pointed shape or a truncation thereof is disclosed.
If the technique disclosed in the Japanese Patent Application Laid-Open (kokai) No. 10-203898 is used, a silicon single crystal free from dislocation can be grown without lessening a diameter of the crystal to be grown at the tip end of the seed crystal to 5 mm or less. Therefore, it has an advantage in growing a crystal having a large diameter or a heavy crystal.
However, in the technique of manufacturing a silicon single crystal described in the aforementioned Japanese Patent Application Laid-Open (kokai) No. 10-203898, there is a problem of how to adjust operation conditions so as not to generate slip dislocations when the seed crystal is brought into contact with a silicon melt. Even if a seed crystal having a tip end with a sharp-pointed shape or a truncation thereof is used, in the case where a difference of temperatures between the seed crystal and the silicon melt when the seed crystal comes into contact with the silicon melt is larger than required, innumerable slip dislocations are introduced into the seed crystal and it is impossible to eliminate the slip dislocations without necking. Moreover, many points to be studied in terms of operation has remained, for example, slip dislocations are introduced into the seed crystal if the temperature of a silicon melt largely changes even while the tip end of the seed crystal is dipped into the silicon melt to the desired diameter and so on.
As for the silicon wafer, a silicon wafer having a plane orientation of (100) or (111) in a main surface on which a semiconductor device is to be formed has been mainly utilized in view of physical characteristics and advantages in the processes of growing a crystal and fabricating a semiconductor device. However, since transport of carriers when forming a semiconductor device considerably depends on a crystal orientation, in recent years, with the aim of enhancing the working speed of the semiconductor device, a silicon wafer having a plane orientation of (110) in which high switching speed is expected starts to attract attention (Nikkei Microdevices, the February 2001, No. 188, Nikkei BP Inc., published on Feb. 1, 2001).
In order to obtain the silicon wafer having a plane orientation of (110), it is possible to utilize a method in which a silicon single crystal having a crystal orientation of <100> or <111> is processed so that a (110) plane may be a main surface of a wafer, or a method in which a silicon single crystal having a crystal orientation of <110> is grown from the first and processed into a silicon wafer. However, the former method in which a silicon wafer having a plane orientation of (110) in a main surface thereof is manufactured from a single crystal having a crystal orientation of <100> or <111> requires oblique cutting of the cylindrical crystal so that the main surface may be a (110) plane. Therefore, it is not an efficient method for industrial mass production of silicon wafers because, in order to obtain an approximately circular silicon wafer to be a substrate for a standard semiconductor device, stock removal for making its shape becomes great loss and time for processing is long.
To the contrary, in the method in which a single crystal having a crystal orientation of <110> is grown from the first and a silicon wafer with a main surface of a (110) plane is manufactured therefrom, if the silicon single crystal is sliced perpendicularly to the direction of the pulling axis as in the case of manufacturing silicon wafers having other plane orientations, and mirror-polishing is performed, a silicon wafer having a main surface of a (110) plane can be obtained. According to this method, since processing after pulling a single crystal can be performed as in the case of a wafer having a plane orientation of (100) or (111), grinding loss generated when making a shape of a wafer and processing time for making the shape can be minimized. Therefore effective wafer processing without loss can be performed.
However, this method has a problem in growing a silicon single crystal having a crystal orientation of <110>.
Namely, in the case of a crystal having a crystal orientation of <100> or <111>, since slip dislocations caused in a seed crystal by thermal shock are introduced at an angle of about 50–70° to a crystal growth interface, the slip dislocations can be taken out (eliminated) from a crystal to be grown by decreasing a diameter of the crystal to about 3–5 mm. However, in the case of a crystal having a crystal orientation of <110>, because slip dislocations are introduced in the direction approximately perpendicular to a crystal growth interface, it is difficult to eliminate the slip dislocations easily from the crystal to be grown. Consequently, there is a need to grow a silicon single crystal using a method in which a diameter of a neck portion is extremely lessened to less than 2 mm as described in Japanese Patent Application Laid-open (kokai) No. 9-165298 etc., or a special method in which, for example, slip dislocations are taken out by forming multi-step concavity and convexity at a neck portion through repeated operation of lessening a diameter of a neck portion to about 3–5 mm and then enlarging the diameter.
Particularly in the case of growing a silicon single crystal having a crystal orientation of <100> or <111>, if a few slip dislocations are generated at a tip end of a seed crystal by thermal shock, the slip dislocations can be eliminated by virtue of using a seed crystal having a tip end with a sharp-pointed shape or a truncation thereof while the seed crystal is dipped to the desired diameter. However, in the case of a crystal having a crystal orientation of <110>, because slip dislocations are introduced in the direction approximately perpendicular to a melting surface of a seed crystal as described above, it is extremely difficult to eliminate even a few slip dislocations once introduced into the seed crystal.
Thus, in order to grow a silicon single crystal having a crystal orientation of <110> by use of a seed crystal having a tip end with a sharp-pointed shape or a truncation thereof without using Dash Necking method, there is a need to form further adequate operation conditions than the case of growing a silicon single crystal having a crystal orientation of <100> or <111>.
Moreover, also in the case of growing a silicon single crystal having a crystal orientation of <110>, for production of a heavy silicon single crystal with a large diameter, if a neck portion is formed by Dash Necking method to eliminate slip dislocations and further the minimum diameter of the neck portion is lessened to about 2–3 mm to ensure elimination of dislocations, it is hardly possible to pull a silicon single crystal with a large diameter of 200 mm or more and weight of 100 kg or more. In order to support such a heavy silicon single crystal with a large diameter and pull it, there is a need that a diameter of a crystal formed at a tip end of a seed crystal is kept to 5 mm or more even at a portion with minimum diameter.